When certain chemical or biological compounds are brought into contact with the circulatory system of humans or other mammals, they cause a systemic response known as the inflammatory response or inflammation. The inflammatory response is a defense mechanism to protect the body from infection and/or injury; inflammation increases blood flow to the site of infection or injury, bringing necessary fluids, proteins, and white blood cells (leukocytes) to help in the healing process. For example, one symptom associated with the inflammatory response is an elevation in body temperature or fever, which functions as a defense mechanism to pathogens that cause overheating. The inflammatory response may be associated with a variety of “flu-like” symptoms including fever, chills, fatigue, headaches, loss of appetite, and muscle stiffness. A chemical or biological compound that triggers fever has historically been referred to as a “pyrogen” or a “pyrogenic” compound, referring to the fever response which such compounds may cause. Some chemical or biological compounds, however, are generally pro-inflammatory and may or may not cause fever as part of the inflammatory response that they cause.
In some cases, depending on the sensitivity of an individual and the type and concentration of pyrogen the individual is exposed to, an individual can develop life-threatening shock-like symptoms after exposure to a pyrogen. Medical products which can be inhaled, injected, or infused and medical devices such as membranes or implanted materials pose a particular risk of pyrogenicity. Even nutrients can represent a risk of pyrogenicity. Pyrogens contained in medical products and nutrients are referred to as exogenous pyrogens; in contrast, endogenous pyrogens are messenger compounds of the immune system that mediate an individual's inflammatory response to exogenous pyrogens. In addition to the pyrogenic nature of a product itself or by-products of its production, contamination of the product can often cause pyrogenicity. Pyrogenicity due to contamination of a product can be caused by any one of a diverse group of pyrogens derived from bacteria, viruses, fungi, or even from the host. This problem can persist even if the product is “sterilized” by heat or chemical methods; a commonly encountered pyrogenic compound, bacterial endotoxin (consisting largely of lipopolysaccharide (LPS) from the cell wall of Gram-negative bacteria), can remain after the bacteria are killed. Thus, pyrogen testing of various pharmaceuticals, nutrients, and medical products for parenteral application is necessary in order to ensure the safety of such products.
Usually, compounds which act as a pyrogen do so by stimulating the production of endogenous pyrogens, such as prostaglandins and proinflammatory cytokines, in monocytes after contact with tissue, cells, or body fluids. It is these endogenously produced pyrogens which mediate the inflammatory response in the affected organism. The most important and well-known of these endogenous pyrogens are the cytokines interleukin-1 (IL-1), interleukin-6 (IL-6), interleukin-8 (IL-8) and tumor necrosis factor (TNF) and the low molecular weight lipid mediator prostaglandin E2 (PGE2). These compounds are routinely assayed by ELISA, or enzyme-linked immunosorbent assays (for IL-1, IL-6, or TNF), and EIA, or enzyme immunoassay (for PGE2).
In order to avoid a pyrogenic reaction and ensure the safety of any drug or pharmaceutical product administered parenterally, pyrogenic contamination must be monitored to identify individual lots that are contaminated with bacterial contaminants. Two animal-based Pharmacopial methods, the Limulus amebocyte lysate (LAL) test, also referred to as the bacterial endotoxins test (BET), and the rabbit pyrogen test, are currently routinely used to monitor pyrogen contamination in mass-produced pharmaceutical products.
The rabbit test is an in-vivo test which consists of injecting a statistically significant number of rabbits with the sample compound and observing the average rise in body temperature elicited in the test animals. Although the rabbit test is responsive to a wide spectrum of pyrogenic agents, including non-endotoxin pyrogens, the rabbit test has a relatively low sensitivity (ng endotoxin/ml) compared to other pyrogen tests (pg endotoxin/ml for the LAL test). In addition, the correlation of pyrogenic responses to compounds between species is, at best, approximate. It has been documented, for instance, that the dose of bacterial endotoxin eliciting a pyrogenic response varies as much as 10,000 fold between species. The relative insensitivity, poor quantitative results, variability between rabbit species, and ethical issues involved in animal testing have made the rabbit test disfavored in recent years.
In contrast to the rabbit test, which detects a wide range of pyrogens, the LAL test only detects endotoxin pyrogens. Bacterial endotoxin, e.g., lipopolysaccharide (LPS), which comes from the cell wall of Gram-negative bacteria, is one of the best described pyrogenic compounds (Moltz et al., Neurosci. Biobehay. Rev., 1993, 17, 237-269; Tilders et al., Psychoneuroendocrinology, 1994, 19, 209-232; Rothwell, Crit. Rev. Neurobiol., 1994, 8, 1-10; Zeisberger and Roth, Neuropsychobioloqy, 1993, 28, 106-109). It was therefore thought to be generally useful to replace expensive and time consuming rabbit experiments with a direct LAL test for bacterial endotoxin. This approach has obvious limitations. The LAL test is a very sensitive in-vitro test; however, it only detects endotoxins from Gram-negative bacteria and gives false negative results with certain products which can still stimulate monocytes to make pyrogenic cytokines. The LAL test is also susceptible to interference by, for example, high protein levels of test substances or by glucans (Roslansky and Novitsky, J. Clin. Microbiol., 1991, 29, 2477; Fennrich et al., Dev. Biol. Stand., 1999, 101, 131). On the other hand, the Limulus test is so sensitive that it is easily prone to false positive results due to impurities that are not relevant to product quality (Fujiwara et al., Yakuqaku Zasshi, 1990, 110, 332-340).
Thus, a need existed for a non-animal-based test system that is characterized by high sensitivity, high specificity, and the ability to detect a wide range of pyrogens. With this intent and with an improved understanding of the human inflammatory response, test systems based on the in vitro activation of human monocytes were developed. Some 20 years ago, researchers used peripheral blood mononuclear cells (PBMC) to detect endotoxin by monitoring the release of pyrogenic cytokines. (Dinarello et al., J. Clin. Microbiol., 1984, 20, 323; Duff and Atkins, J. Immunol. Methods, 1982, 52, 323). Since then, several different test systems using different sources of human monocytes, including human peripheral whole blood (WB), PBMCs, or monocytic cell lines, such as MONOMAC-6 (MM6) (Ziegler-Heitbrock et al., Int. J. Cancer, 1988, 41, 456) or THP-1 (Tsuchiya et al., Int. J. Cancer, 1980, 26, 171), and various readouts, including the pyrogenic cytokines tumor necrosis factor alpha, TNF-α, IL-6, IL-1β, and the non-pyrogenic metabolite neopterin (Hartung et al., The Report and Recommendations of ECVAM Workshop 43, 2001, 29, 99; Poole and Gaines Das, Eur J Parenteral Sciences, 2001, 6, 63; Poole et al., J. Immunol. Methods, 2003, 274, 209; Gaines Das et al., J Immunol Methods, 2004, 288, 165), have been developed. Recently, in a collaborative European study, the Human(e) Study, six of the most prominent monocyte activation tests, each using a different combination of the above described cell sources and readouts were evaluated for the ability to detect endotoxin in medical products spiked with various concentrations of pure endotoxin.
In five of the six tests, the cells were cultured on 96-well polystyrene plates with flat bottomed wells (Hoffmann et al., J. Immunol. Methods, 2005, 298, 161). In the sixth test (based on Fennrich et al., Dev. Biol. Stand., 1999; Fennrich et al., ALTEX, 1999; Hartung et al., 2001), Eppendorf centrifuge tubes (1.2 ml) made of polystyrene, with conical bottoms, were used for the greater part of the evaluation and, part way into the study, polypropylene tubes (1.5 ml), with round bottoms, were substituted for the polyethylene tubes by Charles River Laboratories, the manufacturer of the Endosafe® In vitro Pyrogen Test (IPT) kit used in the study. This substitution was not reported to have any significant effect on the test, and the polypropylene tubes themselves were thereafter replaced with flat-bottomed polystyrene 96-well plates, when Charles River Laboratories modified the Endosafe® IPT for reduced volumes. The source of monocytes in this test was whole blood and the readout was IL-1β. Whole blood was incubated with various drugs spiked with different concentrations of endotoxin.
Carlin and Viitanen disclose a monocyte activation test, based on whole blood or MONOMAC-6 cells as the source of monocytes and IL-6 as the readout, for evaluating the inherent pyrogenicity of the vaccine Infanrix, which contains Gram-positive and Gram-negative antigens, including various non-endotoxin pyrogens, i.e., Diphtheria toxoid, Pertussis toxoid, and Tetanus toxoid (Pharmeuropa, 2003, 15, 3, 418-423). Cells were cultured at low cell density in endotoxin-free Eppendorf Bio-Pure grade tubes of undisclosed composition. The pyrogenicity of the vaccine was not the result of contamination of the vaccine during its manufacture or storage. Hartung and Wendel disclose a monocyte activation test, based on whole blood as the source of monocytes and IL-1β as the preferred readout, for detecting endotoxin and non-endotoxin pyrogens in their pure forms, i.e., LPS from Salmonella abortus equi, streptolysin O (SLO) from Streptococcus pyrogens, and muramyl dipeptide (MDP) (Hartung and Wendel, In Vitro Toxicology, 1996, 9, 4, 353-359; U.S. Pat. No. 5,891,728). Cells were cultured in polypropylene tubes.
Yamamoto et al. disclose a monocyte activation test, based on whole blood or cells from different human cell lines, the 28SC cell line being preferred, as the source of monocytes/monocytic cells and IL-6 as the preferred readout, for detecting endotoxin pyrogens (Jpn. J. Infect. Dis., 2003, 56, 93-100). The endotoxin test is said to predict the possibility of an in vivo synergism between endotoxin and a parenteral drug, particularly interferon, such that the drug augments the pyrogenic effects of the endotoxin. Cells were cultured with either endotoxin in its pure form or a mixture of endotoxin in its pure form and human interferon. Cell line cells were cultured in polystyrene 96-well plates with flat-bottomed wells. Blood cells were cultured in tubes of unspecified material.
Nakagawa et al. describe a monocyte activation test, based on whole blood or MM6-CA8 cells (a subclone of MONOMAC-6) as the source of monocytes and IL-6 as the preferred readout, for detecting endotoxin and non-endotoxin pyrogens in their pure forms, i.e., LPS from E. coli O55:B5 and insoluble peptidoglycan (PG) derived from S. aureus (Clinical and Diagnostic Laboratory Immunol., 2002, 9, 3, 588-597). MM6-CA8 cells were cultured in polystyrene 96-well plates. Blood cells were cultured in polypropylene tubes; the volumes used (225 μL of blood, 25 μL of test solution, 750 μL of saline) precluded the used of standard 96-well plates (250 μL/well).
Recently, various sources indicate that polypropylene is to be avoided in pyrogen testing. For example, Charles River Laboratories recommends avoiding polypropylene because the hydrophobic nature of a polypropylene surface could result in the adsorption of endotoxin on such a surface due to the hydrophobic domains associated with the Lipid A component of LPS (Charles River Laboratories, Endosafe Times, September 2004). Harlan Sera-Lab, a manufacturer of biosafety tests, states that polypropylene tubes can interfere with the LAL assay (Harlan Sera-Lab, 2004 Catalogue). And, the European Dialysis and Transplant Nurses Association as well as the European Renal Care Association recommend avoiding polypropylene and using polystyrene in endotoxin testing because polystyrene does not normally adsorb endotoxin (EDTNA/ERCA Guidelines).
Thus, a need exists for a non-animal-based pyrogen test, characterized by high sensitivity, high specificity, and the ability to detect a wide range of pyrogenic contaminants in medical products.